Wood comprises several different components: cellulose; hemicelluloses, such as xylan; lignin and extractives. During chemical pulping for instance in a kraft, i.e. sulphate, pulp mill the xylan chain forms side groups called hexenuronic acids (HexAs) which are unsaturated sugars. The amount of HexAs varies from pulp to pulp, because different wood species contain different amounts of xylan, which can be transformed into HexAs during the cooking process. Also, cooking parameters contribute to different amounts of HexAs.
The process of kraft pulping comprises alkaline cooking and bleaching, and it begins with wood handling where wood is debarked and made into chips. The chips are screened so fine material and oversized chips are eliminated. The chips are then fed to a digester where they first are treated with steam and then with cooking liquid, while the temperature is raised to the desired cooking temperature. When desired rate of delignification is achieved, cooking is interrupted and the content in the digester is moved to a blow tank and onwards to a screener. After the pulp is screened it is washed several times and pumped to the following delignification stage, i.e. initial bleaching. The cooking chemicals are recovered in the chemical recovery plant.
The main target for chemical pulping process is delignification in order to liberate the fibres without harming them. Alkaline delignification occurring during cooking is alkaline hydrolyses of phenol ether bonds that make lignin soluble. Phenols are weak acids that dissociate in alkali environment (pH>10). The lignin will be partly demethylated by nucleophilic attack of sulfide ions on methoxyl groups in lignin. Bleaching of the obtained pulp comprises typically a number of discrete steps or stages. In the oxygen delignification, which may occur either as pre-bleaching or bleaching step, more lignin is dissolved and washed away. This is also the case in the different following bleaching stages; peroxide bleaching, ozone bleaching and chlorine dioxide bleaching. Finally the pulp is moved to the papermaking process in integrated pulp and paper mills or it is traded as market pulp after the drying machine where it is dried, cut and packed for further transportation to paper mills.
Oxygen delignification occurring in pre-bleaching or bleaching step may comprise only one stage, but usually the process is carried out in a two-stage system with or without washing between the stages. In typical one stage oxygen delignification system the unbleached pulp is washed in the filtrate from the post-oxygen washer before it is charged with NaOH or oxidized white liquor. The pulp is preheated in a low-pressured steam mixer before it is transferred by a medium consistency pump to the high-shear, medium-consistency mixer. Oxygen is added to the mixer and the oxygen delignification process begins.
The first stage after oxygen delignification may be a delignification stage using chlorine dioxide to dissolve lignin. The typical following alkaline extraction stage (EOP) stage is an alkaline extraction stage enhanced with the oxidizing agents: oxygen and peroxide.
Alkaline oxygen and peroxide bleaching stages do not affect the HexA content in pulp. Chlorine dioxide and ozone on the other hand have a great impact on the HexA content and will react with the HexA groups in the pulp. HexAs are consumed in the chlorine dioxide stage forming unchlorinated and chlorinated dicarboxylic acids. The HexAs thus consume bleaching chemicals (electrophilic bleaching agents) and also increase brightness reversion of fully bleached pulps.
Moreover, the HexAs also bind heavy metal ions and increase the problems with non-process elements (NPEs) which will lead to an increase in deposits in the bleaching stages. This is why it is in interest to remove these components from the pulp before the bleaching stages. In that case a lower chemical batch can be used in each delignification or bleaching stage and higher brightness stability can be achieved. The kappa number, that is a measure of lignin content in pulp, is also affected by HexAs. HexAs consume potassium permanganate that is one of the reactants used in the kappa number analysis. Permanganate reacts with carbon-carbon double bonds in the lignin structure but HexAs also contribute to the consumption because of its carbon-carbon double bond.
The hot acid stage (A-stage, at pH 3, temperatures of 50-90° C. and retention time of 1-3 hours), that is disclosed in U.S. Pat. No. 6,776,876 and the hot chlorine dioxide bleaching (at temperatures 60-90° C.) disclosed in WO 2008/044988 are two methods to eliminate HexAs that are used today. Both these methods leave residual HexAs in the pulp, increase the retention time in the bleaching lines, increase the costs of effluent treatment, reduce the amount of charged groups on the fibre surface and reduce the fibre strength properties. WO 2012/022840 suggests carrying out the oxygen treatment stage in the presence of at least one perbenzoic acid, in order to decrease the amount of hexenuronic acid.
An object of the present invention is to reduce or eliminate hexenuronic acids (HexA) from lignocellulosic pulps and/or improve/increase the pulp brightness. Another object is to increase the pulp brightness e.g. without reducing the content of hexenuronic acids in the pulp.